The present invention relates to a solid electrolyte type gas sensor and, more particularly, to a sensor operable at or about ambient temperature.
Solid electrolyte sensors at present comprise a detection electrode and a reference electrode disposed on opposite sides of a solid electrolyte acting as an ionic conductor. Generally, in detecting gaseous components present in an atmosphere using a solid electrolyte sensor, an ionic conductor is used in which specific ions are mobile and sometimes in combination with this specific ion conductor, which is used as the solid electrolyte, another compound containing the specific ions is used as a detection material and is covered by an electrocatalytic material, such as platinum.
JP-A-62-142266 describes an NOx sensor comprising a gold electrode on an insulating substrate, a solid RbAg4I5 electrolyte, and a gold detection electrode with AgNO3 deposited on it to act as an electrocatalyst. Such a sensor requires a reference gas in contact with the reference electrode to provide a stable output potential. The NOx sensors of JP-A-61-271447 and JP-A-61-184450 have similar requirements. JP-A-62-207952 describes a halogen gas sensor having a reference electrode comprising a metal halide, metal, and a metal ion-conductive solid electrolyte. The cell electrolyte is a halide-containing metal ion-conductive solid electrolyte, and the detection electrode is said to be a metal halide-metal ion-conductive solid electrolyte mixture. The detection electrode does not have an electrocatalyst layer; the operating temperature of this sensor is 150xc2x0 C.
A previously proposed gaseous carbon dioxide sensor uses, for example, a sodium ionic conductor such as xcex2-alumina (general formula: Na2O.nAl2O3, n=5-11) or NASICON(general formula: Na1xe2x88x92xZr2P3xe2x88x92xSixO12). In this case, a platinum gauze covered with sodium carbonate or the like is used as a detection electrode.
A typical reference electrode comprises gold or platinum alone or these metals covered with sodium carbonate or the like, maintained in contact with reference atmosphere of standard air or gaseous carbon dioxide. Accordingly, gaseous carbon dioxide at the concentration to be measured is in contact with the detection electrode but not with the gas reference electrode on the opposite side.
The sensor is heated during operation usually to a constant temperature between 400xc2x0 C. and 600xc2x0 C., a flux of sodium ions being caused to move to the detection electrode corresponding to the partial pressure of the gaseous carbon dioxide in a gas to be detected which is in contact with the detection electrode. This gives rise to a sodium ion gradient between the electrodes and the concentration of the gaseous carbon dioxide can be deduced by measuring the potential difference associated with this gradient.
However, in the case of the existing gaseous carbon dioxide sensors using sodium carbonate as the detection material for the detection electrode and using NASICON for the ionic conductor as described above, the potential difference is highly sensitive to the moisture content of the gas to be detected.
In order to overcome this effect, a gaseous carbon dioxide detection sensor comprising a detection electrode and a reference electrode on both sides of an ionic conductor with a mixture of one mole of an alkali metal carbonate and more than one mole of an alkaline earth metal carbonate such as BaCO3 has been suggested as the material for the detection electrode. For such a sensor the variation of potential difference with carbon dioxide content is less affected by moisture in the gas to be measured.
A shortcoming of even this improved sensor is the operating temperature, which requires a heater. This elevated operating temperature is also a potential cause of accelerated breakdown of the sensor structure and problems associated with oxidation of electrocatalysts and contacts. Furthermore, in many cases, a reference gas is required which complicates the cell design and affects portability.
The present invention overcomes the foregoing problems in the prior art and provides a gas, especially carbon dioxide, detection sensor capable of measuring the concentration of a gas without the need for high temperature operation. Conveniently, the sensor may be operated at a controlled ambient or moderately elevated temperature, for example up to 70xc2x0 C., to avoid any possible error resulting from ambient temperature variation. In one embodiment the use of a reference gas is avoided.
The present invention provides in a first aspect a gas sensor containing an electrolytic cell comprising a reference electrode, a detection electrode, and a solid electrolyte, the detection electrode comprising an electrocatalyst and a silver salt the anion of which corresponds to the gas to be detected, the solid electrolyte being capable of transmitting silver ions, and the reference electrode comprising metallic silver in contact with the electrolyte, the free face of the silver being sealed. A cell of this type is referred to below as one of the closed gas type.
As the gas to be detected, there may be mentioned, for example, sulphur trioxide, sulphur dioxide, nitrogen oxides (Nox, or specific nitrogen oxides), hydrogen sulphide and halogen, especially chlorine, or pseudohalogen, for example, cyanogen. The corresponding anion in the detection electrode is sulphate, sulphite, an oxyacid of nitrogen, and halide, especially chloride, or pseudohalide, for example, cyanide. More especially, and preferably, the gas to be detected is carbon dioxide and the corresponding anion is a carbonate.
The reference electrode is advantageously a metallic silver sheet in contact with the silver ion conductor.
In a second aspect the present invention provides a carbon dioxide sensor containing an electrolytic cell comprising a reference electrode, a detection electrode, and a solid electrolyte, the detection electrode comprising an electrocatalyst and silver carbonate, and the solid electrolyte being capable of transmitting silver ions.
In a first embodiment of the second aspect of the invention, a reference electrode having the same characteristics as set out below for the detection electrode is advantageously used, and preferably the reference electrode is of the same materials, and in the same proportion, as is the detection electrode.
Advantageously, in the first embodiment of the second aspect an atmosphere of a known constitution (a reference atmosphere) is maintained at the free face of the reference electrode. The reference atmosphere is advantageously air at atmospheric pressure with a known and constant CO2 partial pressure. A cell of this type is referred to below as one of the open gas type.
In a second embodiment of the second aspect, the reference electrode is sealed. In this second embodiment the reference electrode is advantageously metallic silver in contact with the silver ion conductor. A cell of this type is referred to below as one of the closed gas type.
In each aspect, the detection electrode, in addition to a silver salt of an acid corresponding to the gas to be detected (carbonate in the second aspect), advantageously also contains a component that enhances the conductivity of the electrode. This component is advantageously an ionic conductor, and one through which silver ions may pass, preferably at room temperature, and may comprise a silver salt or double salt, for example, silver iodide or, advantageously, a silver rubidium iodide, especially Ag4RbI5, or a silver mercury iodide. Advantageously, the proportion of conductivity enhancer is adequate to provide a sufficiently high ionic conductivity.
The detection electrode advantageously also comprises a binder, e.g., polytetrafluorethylene (PTFE), in a proportion sufficient to render the conductive materials coherent but without adversely affecting conduction to a deleterious extent.
As solid electrolyte, there is advantageously used an ionic conductor, for example and preferably one mentioned above as the conductivity enhancer in the detection electrode, and more preferably the same ionic conductor as is used in that electrode. The electrolyte advantageously also comprises a binder, conveniently the same as that used in the electrode.
In each aspect, the detection electrode comprises an electrocatalyst advantageously in the form of a layer permeable to the gas to be detected, for example, platinum. A similar catalyst is advantageously provided on the free face of the reference electrode in the first embodiment of the second aspect.
Advantageously, platinum leads are provided to the detection electrode and in the first embodiment of the second aspect to the reference electrode. Although the reference electrode in the first aspect and the second embodiment of the second aspect may be provided with a platinum lead, other conductive leads may be used.
The apparatus in which the cell is placed is described in greater detail below. The cell itself is conveniently in the form of a disk.
The present invention more especially concerns a solid electrolyte type carbon dioxide sensor based on silver compounds which have been shown to have a good response to carbon dioxide concentration at room temperature. In particular, the present invention advantageously includes the presence as electrolyte, conductivity enhancer, or both, of silver rubidium iodide which is an excellent ionic conductive material at room temperature (the ionic specific conductivity of silver rubidium iodide is as high as 0.24(ohm cm)xe2x88x921 at 25xc2x0 C.) which allows it to be used in the detection of carbon dioxide concentrations at ambient or near ambient temperatures.
Two different embodiments of carbon dioxide sensor based on silver compounds will be described in more detail, and will be referred to hereinafter for convenience as the open gas type and the gas tight type.
The open gas type of carbon dioxide gas sensor comprises the following electrochemical cell:
CO2, O2,N2|Pt|Ag2CO3+Ag4RbI5|Ag4RbI5|Ag2CO3+Ag4RbI5|Pt|Air
where the right hand electrode is the reference electrode and the left hand electrode is the detecting electrode.
The detecting electrode equilibrium is
Ag2CO3I⇄2Ag++CO2I+1/202I+2exe2x88x92
and the reference electrode equilibrium is
2Ag++CO2II+1/202II+2exe2x88x92⇄Ag2CO3II
The overall equilibrium is thus
Ag2CO3I+CO2II+1/202II⇄Ag2CO3II+CO2I+1/202I
and the cell potential(E) is given by
E=(xcex94Gxc2x0Ag2CO3II+xcex94Gxc2x0CO2I+xc2xdxcex94Gxc2x0O2Ixe2x88x92xcex94Gxc2x0Ag2CO3Ixe2x88x92xcex94Gxc2x0CO2IIxe2x88x92xc2xdxcex94Gxc2x0O2II)/2Fxe2x88x92(RT/2F)xc3x97lnPco2
where xcex94Gxc2x0i is the standard free energy of formation for species i, F the Faraday constant, R the gas constant, T the absolute temperature, and Pco2 the partial pressure of CO2. It is assumed in the equation that the activity of Ag2CO3 is unity. The silver rubidium iodide does not participate in electrode or cell reactions. It is present only as an ionic conductor.
Secondly, the gas tight type of carbon dioxide gas sensor comprises the following electrochemical cell:
CO2, O2,N2|Pt|Ag2CO3+Ag4RbI5|Ag4RbI5|Ag
The detecting electrode equilibrium is
Ag2CO3⇄2Ag++CO2+1/202+2e
and the reference electrode equilibrium is
2Ag++2exe2x88x92⇄2Ag
The overall equilibrium is thus
Ag2CO3⇄2Ag+CO2+1/202
and the cell potential (E) is given by
E=(2xcex94Gxc2x0Ag+xcex94Gxc2x0CO2+1/2xcex94Gxc2x0O2xe2x88x92xcex94Gxc2x0Ag2CO3)/2Fxe2x88x92(RT/2F)xc3x97lnPco2
where the symbols have the meanings give above. It is assumed in the equation that the activities of Ag2CO3 and Ag are unity.
In this gas tight type carbon dioxide gas sensor the detecting electrode is open to the test gas but the silver reference electrode is sealed, and therefore in this gas tight type sensor there is no need to supply a separate reference gas.